1. Field of the Invention
This invention relates to the fabrication of a semiconductor laser, and more particularly, to a self-aligned method for fabricating a ridge-waveguide semiconductor laser diode.
2. Description of Related Art
The structure of a common ridge-waveguide laser diode is shown in FIG. 1A. The laser diode includes a substrate 100, such as a N-type substrate, and a first cladding and guiding layer 102, an active layer 104, a second cladding and guiding layer 106, a dielectric layer 108, and a cap layer 110 formed sequentially on the substrate 100. There are a metal layer 112, a P-type metal electrode, located on the cap layer 110, and another metal layer 114, a N-type metal electrode, located underneath the substrate 100. The waveguide structure of the laser consists of the active layer 104, the first cladding and guiding layer 102, and the second cladding and guiding layer 106. Because the refraction index of the active layer 104 is larger than that of these two cladding and guiding layers 102 and 106, the light generated by the recombination of carriers is then confined within the active layer 104.
Nowadays, double heterostructure (DH) is widely used in laser diodes. When the P-type electrode 112 is connected to a positive voltage and the N-type electrode is connected to a negative voltage, a consequent bias is generated. The consequent bias forces electrons from the N-type electrode, and the holes from the P-type electrode flow toward the active layer 104. The potential barrier generated in the active layer 104 resists the passing of those electrons and holes. As a result, the over-populated electrons and holes within the active layer 104 cause population inversion. The recombination of carriers releases light of the same energy and phase, also known as a laser, which is an acronym for light amplification by stimulated emission radiation. In the foregoing ridge-waveguide laser, current can only flow through the surface of the ridge. The dielectric layer located on the sides of the ridge structure guides the light wave efficiently to improve the electro-optic effect.
Referring to FIG. 1B, a ridge-waveguide laser of a double-channel structure 116 is shown. The first cladding and guiding layers 102 and 106 are further divided into cladding layers 102a and 106a, and guiding layers 102b and 106b. Then, as shown in FIG. 1B, an etching process is performed on the laser structure to form the double channel 116. A dielectric layer 108 is formed on the entire structure, and the dielectric layer 108 is patterned to form a contact opening that exposes the top of the cap layer 110. Then, a P-type metal electrode 112 is formed on the top of the substrate 100 and an N-type metal electrode 114 is formed underneath the substrate 100 to accomplish the structure of a ridge-waveguide laser diode.
Even though the foregoing method for forming a ridge-waveguide laser diode is simple, misalignment occurs in the step of forming the contact opening on the ridge structure 120, especially as the dimension of the ridge structure 120 is small. For example, in a case having a contact opening of a 2-.mu.m width on the ridge-structure of a 3-.mu.m width, the alignment tolerance on either side is only 0.5 .mu.m. This is too tiny for existent fabrication processes. Furthermore, because the ridge structure 120 is not entirely covered by the metal layer 112, the resistance of ohmic contact is larger and the thermal radiation is worse. That is, the conventional method for fabricating a ridge-waveguide laser diode cannot provide a convenient and reliable fabrication process, and a high-performance laser diode at the same time.
There are a number of methods to resolve the foregoing problems of fabricating a ridge-waveguide laser, such as U.S. Pat. No. 4,728,628, U.S. Pat. No. 4,830,986, U.S. Pat. No. 5,059,552, U.S. Pat. No. 5,208,183, U.S. Pat. No. 5,474,954, U.S. Pat. No. 5,504,768, and U.S. Pat. No. 5,658,823.
As provided by the U.S. Pat. No. 5,504,768, a method includes forming a P-type metal layer, and using the P-type metal layer as a mask to form the ridge structure and the double channel, forming a dielectric layer on the substrate, and then, forming openings. Since the P-type metal layer covers the entire ridge structure, the problems of overheating and high resistance are resolved. However, the method has an alignment problem during the process of forming a narrow ridge structure.
There is another method described in U.S. Pat. No. 5,474,954 that applies a technique of self-alignment to from a current cutoff layer on the sidewall of the P-type metal for reducing the heat generated during lasing. As the integration of the laser diode is raised, an alignment problem still occurs in the fabrication process, and degrades the process yield.
In U.S. Pat. No. 4,728,628, a method that also uses a metal layer as a mask includes forming a dielectric layer after a ridge structure is formed, forming a P-type metal layer, and then, forming openings. The width of the opening is equal to the sum of the width of the double channel and the width of the ridge structure. A smaller opening whose width is equal to the width of the ridge structure is formed within the foregoing opening, and filled with metal. The method overcomes the alignment problem, but the absence of a dielectric layer on either sidewall of the ridge structure causes problems including peeling of devices and a poor reliability under a high working temperature.
Likewise, in U.S. Pat. No. 5,208,183, a method is provided to fabricate a ridge waveguide laser diode having a very narrow ridge waveguide. Even though the provided method resolves the alignment problem by eliminating critical alignment steps from the fabrication process, other problems such as overheating still exist. In addition, Since current can only flow through a limited cross section, the resistance of ohmic contact on the laser diode is extravagant.
Besides, as described in U.S. Pat. No. 5,658,823, a method is provided to protect the dielectric on either sidewall of the ridge structure. The provided method includes removing only a portion of the photoresist located on the top of the ridge structure and in the mean time, still keeping the photoresist in the double channel. Referring to FIGS. 2 and 3, property curves are used to explain the relationship between the remaining thickness of different photoresist and the exposure time. As shown in FIG. 2. the curve 200 shows the relationship between the remaining thickness of a photoresist AZ1500 and the time exposed under the G-line mask aligner, whose wavelength is about 300 nm and up. The photoresist AZ1500 is entirely removed by just being exposed to the G-line for 10 seconds. In other words, for every two seconds the photoresist AZ1500 is exposed to the G-line, a thickness of a couple thousand angstroms is removed.-line So, it is obvious that the processing rate of the photolithography process that uses AZ1500 and the G-line is too fast to control.
In FIG. 3, the curve 300 shows the relationship between the remaining thickness of a photoresist ODUR1013 and the time exposed under the I-line, whose wavelength is less than 300 nm. About 100 seconds of exposure time are required to remove all the photoresist, at a rate of about 1000-2000 .ANG. per 10 seconds. Even though -linethe removal rate is slower, it is still difficult to control the photolithography process. Therefore, exposing the dielectric on the top of the ridge structure by removing the photoresist thereon is not a very practical method for the task of fabricating a ridge-waveguide semiconductor laser.
According to the foregoing, misalignment always exists in a conventional method for fabricating a ridge-waveguide semiconductor laser. A ridge-waveguide semiconductor laser made by the conventional method has undesirable properties, a large resistance, and overheating problem. Furthermore, the conventional method, such as the one provided by U.S. Pat. No. 4,729,628, also causes short circuit problems.
Moreover, because the thickness of photoresist and the exposure time are very difficult to control precisely, the dielectric located on the sidewalls of the ridge structure is easily removed. As a result, a metal layer formed by the follow-up process may not only cover the top of the ridge structure as designed, it possibly covers the bottom of the double channel as well. The presence of unexpected materials on the bottom of the double channel degrades the current flowing into the device.